1. Field of the Invention
This invention relates generally to apparatus, such as a video tape recorder (VTR) of the so-called "helical-scan type," in which video or other information signals are recorded in successive parallel tracks which are skewed or extend obliquely on a magnetic tape and, more particularly, is directed to an improved tracking control system by which a magnetic head or other transducer in such apparatus is made to accurately scan the track or tracks in which the video or other information signals are recorded.
2. Description of the Prior Art
In a helical-scan VTR, the magnetic tape extends helically about at least a portion of the periphery of a guide drum and is adapted to be moved or advanced in the longitudinal direction of the tape while at least a portion of the guide drum is rotated, and a transducer or magnetic head is mounted on a rotated portion of the guide drum so as to rotate with the latter and thereby repeatedly scan across the tape in a path at an angle to the longitudinal direction of the tape. During the recording operation of the VTR, the angle between the scanning path, and hence each record track, and the longitudinal direction of the tape is dependent on the rotational speed of the rotary head and also the speed at which the magnetic tape is longitudinally advanced. Accordingly, if the speed and direction of advancement of the magnetic tape are not the same during the reproducing operation as during the recording operation, then the scanning path of the magnetic head during reproducing will not precisely follow or coincide with a record track on the tape during each movement of the head across the tape and, accordingly, the recorded video or other information signals may not be completely or accurately reproduced.
Various tracking control or servo systems have been proposed for maintaining correct tracking or scanning of the record tracks by the rotary head. In one such system, means, such as a bi-morph leaf, is provided for deflecting the head in a direction normal to the plane of its rotation, that is, in a direction which is transverse in respect to the direction along each of the record tracks, and the amount of such deflection is electrically controlled during the movement of the head along each track so as to achieve accurate scanning of the latter. In particular, a drive signal is applied to the deflecting means for controlling the latter to align the center of the magnetic head with the center of the track being scanned. The drive signal includes a tracking error signal corresponding to the deviation of the head from the center of the track being scanned and a wobbling or dithering signal which causes the magnetic head to be wobbled transversely with respect to the longitudinal direction of each record track. As a result of the latter wobbling signal, the RF signal reproduced by the head is subjected to amplitude modulation at the wobbling or dithering frequency in order to detect the amount and direction of tracking error and thereby obtain the tracking error signal.
Generally, in order to obtain the tracking error signal with a satisfactory signal-to-noise (S/N) ratio, the amplitude of the wobbling signal is selected so that the amplitude modulated component of the reproduced RF signal has an amplitude which is approximately 5-10% of the amplitude of the reproduced RF signal. However, when the head accurately follows the track to be scanned, the wobbling or dithering motion of the head is detected as the tracking error having a constant frequency component. In such case, it is desirable to reduce the amplitude of the wobbling or dithering signal to increase the tracking accuracy by the head. In other words a reduction in amplitude of the wobbling signal results in a corresponding decrease in the amplitude of the amplitude modulated component of the reproduced RF signal. It should be appreciated that this is contrary to the case where the magnetic head does not accurately scan the track. In the latter case, as previously mentioned, it is desirable to select the wobbling signal so that the amplitude modulated component of the reproduced RF signal has a relatively large amplitude of 5-10% of the amplitude of the reproduced RF signal.
In a copending case by the same inventor, U.S. patent application Ser. No. 06/232,014, filed Feb. 6, 1981, and having a common assignee herewith, it has been proposed to detect the actual deflection of the bi-morph leaf by means of a strain gauge mounted to the surface of the bi-morph leaf. A DC voltage corresponding to such actual deflection is produced and amplitude detected. The amplitude detected voltage is then compared with a reference voltage to produce an error voltage which is used to adjust the level of the wobbling signal from a controllable oscillator.